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Cretaceous
The Cretaceous ( , ), derived from the Latin "creta" (chalk), usually abbreviated K''' for its German translation Kreide (chalk), is a geologic period and system from circa }} to }} million years (Ma) ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the Cenozoic era. It is the last period of the Mesozoic Era, and, spanning 80 million years, the longest period of the Phanerozoic Eon. The Cretaceous was a period with a relatively warm climate, resulting in high eustatic sea levels and creating numerous shallow inland seas. These oceans and seas were populated with now extinct marine reptiles, ammonites and rudists, while dinosaurs continued to dominate on land. At the same time, new groups of mammals and birds, as well as flowering plants, appeared. The Cretaceous ended with a large mass extinction, the Cretaceous–Paleogene extinction event, in which many groups, including non-avian dinosaurs, pterosaurs, and large marine reptiles, died out. The end of the Cretaceous is defined by the K–Pg boundary, a geologic signature associated with the mass extinction which lies between the Mesozoic and Cenozoic Eras. Geology Research history The Cretaceous as a separate period was first defined by a Belgian geologist Jean d'Omalius d'Halloy in 1822, using strata in the Paris Basin and named for the extensive beds of chalk (calcium carbonate deposited by the shells of marine invertebrates, principally coccoliths), found in the upper Cretaceous of western Europe. The name Cretaceous was derived from Latin creta, meaning chalk. The name of the island Crete has the same origin. Stratigraphic subdivisions The Cretaceous is divided into Early and Late Cretaceous epochs or Lower and Upper Cretaceous series. In older literature the Cretaceous is sometimes divided into three series: Neocomian (lower/early), Gallic (middle) and Senonian (upper/late). A subdivision in eleven stages, all originating from European stratigraphy, is now used worldwide. In many parts of the world, alternative local subdivisions are still in use. As with other older geologic periods, the rock beds of the Cretaceous are well identified but the exact ages of the system's top and base are uncertain by a few million years. No great extinction or burst of diversity separates the Cretaceous from the Jurassic. However, the top of the system is sharply defined, being placed at an iridium-rich layer found worldwide that is believed to be associated with the Chicxulub impact crater in Yucatan and the Gulf of Mexico. This layer has been tightly dated at 65.5 Ma.The official geologic timescale of the ICS (in 2008) gives 65.5 Ma as upper boundary of the Cretaceous, new callibrations by Kuiper et al. (2008) yield 65.9 Ma Rock formations , from the Maastrichtian of Dutch Limburg, by Dutch geologist Pieter Harting (1866).]] The high eustatic sea level and warm climate of the Cretaceous meant a large area of the continents was covered by warm shallow seas. The Cretaceous was named for the extensive chalk deposits of this age in Europe, but in many parts of the world, the Cretaceous system consists for a major part of marine limestone, a rock type that is formed under warm, shallow marine circumstances. Due to the high sea level there was extensive accommodation space for sedimentation so that thick deposits could form. Because of the relatively young age and great thickness of the system, Cretaceous rocks crop out in many areas worldwide. Chalk is a rock type characteristic for (but not restricted to) the Cretaceous. It consists of coccoliths, microscopically small calcite skeletons of coccolithophores, a type of algae that prospered in the Cretaceous seas. In northwestern Europe, chalk deposits from the Upper Cretaceous are characteristic for the Chalk Group, which forms the white cliffs of Dover on the south coast of England and similar cliffs on the French Normandian coast. The group is found in England, northern France, the low countries, northern Germany, Denmark and in the subsurface of the southern part of the North Sea. Chalk is not easily consolidated and the Chalk Group still consists of loose sediments in many places. The group also has other limestones and arenites. Among the fossils it contains are sea urchins, belemnites, ammonites and sea reptiles such as ''Mosasaurus. In southern Europe, the Cretaceous is usually a marine system consisting of competent limestone beds or incompetent marls. Because the Alpine mountain chains did not yet exist in the Cretaceous, these deposits formed on the southern edge of the European continental shelf, at the margin of the Tethys Ocean. Stagnation of deep sea currents in middle Cretaceous times caused anoxic conditions in the sea water. In many places around the world, dark anoxic shales were formed during this interval.See Stanley (1999), pp. 481–482 These shales are an important source rock for oil and gas, for example in the subsurface of the North Sea. Paleogeography During the Cretaceous, the late-Paleozoic-to-early-Mesozoic supercontinent of Pangaea completed its tectonic breakup into present day continents, although their positions were substantially different at the time. As the Atlantic Ocean widened, the convergent-margin orogenies that had begun during the Jurassic continued in the North American Cordillera, as the Nevadan orogeny was followed by the Sevier and Laramide orogenies. in the late Cretaceous period]] Though Gondwana was still intact in the beginning of the Cretaceous, it broke up as South America, Antarctica and Australia rifted away from Africa (though India and Madagascar remained attached to each other); thus, the South Atlantic and Indian Oceans were newly formed. Such active rifting lifted great undersea mountain chains along the welts, raising eustatic sea levels worldwide. To the north of Africa the Tethys Sea continued to narrow. Broad shallow seas advanced across central North America (the Western Interior Seaway) and Europe, then receded late in the period, leaving thick marine deposits sandwiched between coal beds. At the peak of the Cretaceous transgression, one-third of Earth's present land area was submerged.Dougal Dixon et al., Atlas of Life on Earth, (New York: Barnes & Noble Books, 2001), p. 215. The Cretaceous is justly famous for its chalk; indeed, more chalk formed in the Cretaceous than in any other period in the Phanerozoic.Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6 p. 280 Mid-ocean ridge activity—or rather, the circulation of seawater through the enlarged ridges—enriched the oceans in calcium; this made the oceans more saturated, as well as increased the bioavailability of the element for calcareous nanoplankton.Stanley, pp. 279–81 These widespread carbonates and other sedimentary deposits make the Cretaceous rock record especially fine. Famous formations from North America include the rich marine fossils of Kansas's Smoky Hill Chalk Member and the terrestrial fauna of the late Cretaceous Hell Creek Formation. Other important Cretaceous exposures occur in Europe (e.g., the Weald) and China (the Yixian Formation). In the area that is now India, massive lava beds called the Deccan Traps were erupted in the very late Cretaceous and early Paleocene. Climate The Berriasian epoch showed a cooling trend that had been seen in the last epoch of the Jurassic. There is evidence that snowfalls were common in the higher latitudes and the tropics became wetter than during the Triassic and Jurassic.The Berriasian Age Glaciation was however restricted to alpine glaciers on some high-latitude mountains, though seasonal snow may have existed farther south. Rafting by ice of stones into marine environments occurred during much of the Cretaceous but evidence of deposition directly from glaciers is limited to the Early Cretaceous of the Eromanga Basin in southern Australia.Alley, N.F. and Frakes, L.A. 2003. "First known Cretaceous glaciation: Livingston Tillite, South Australia". Australian Journal of Earth Science '''50:134–150.Frakes, L.A. and Francis, J. E. 1988. "A guide to Phanerozoic cold climates from high latitude ice rafting in the Cretaceous". Nature 333:547–549. After the end of the Berriasian, however, temperatures increased again, and these conditions were almost constant until the end of the period. This trend was due to intense volcanic activity which produced large quantities of carbon dioxide. The production of large quantities of magma, variously attributed to mantle plumes or to extensional tectonics, further pushed sea levels up, so that large areas of the continental crust were covered with shallow seas. The Tethys Sea connecting the tropical oceans east to west also helped in warming the global climate. Warm-adapted plant fossils are known from localities as far north as Alaska and Greenland, while dinosaur fossils have been found within 15 degrees of the Cretaceous south pole.Stanley, pp. 480–2 A very gentle temperature gradient from the equator to the poles meant weaker global winds, contributing to less upwelling and more stagnant oceans than today. This is evidenced by widespread black shale deposition and frequent anoxic events.Stanley, pp. 481–2 Sediment cores show that tropical sea surface temperatures may have briefly been as warm as 42 °C (107 °F), 17 °C ( 31 °F) warmer than at present, and that they averaged around 37 °C (99 °F). Meanwhile deep ocean temperatures were as much as 15 to 20 °C (27 to 36 °F) higher than today's."Warmer than a Hot Tub: Atlantic Ocean Temperatures Much Higher in the Past" PhysOrg.com. Retrieved 12/3/06.Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth: An Introduction to Physical Geology. 3rd ed. New York: John Wiley & Sons, Inc., 1995. ISBN 0-471-59549-7. p. 557 Life '' (Here: Modern Araucaria araucana in Chile).]] Flora Flowering plants (angiosperms) spread during this period, although they did not become predominant until the Campanian stage near the end of the epoch. Their evolution was aided by the appearance of bees; in fact angiosperms and insects are a good example of coevolution. The first representatives of many leafy trees, including figs, planes and magnolias, appeared in the Cretaceous. At the same time, some earlier Mesozoic gymnosperms like conifers continued to thrive; pehuéns (monkey puzzle trees, Araucaria) and other conifers being notably plentiful and widespread. Some fern orders such as GleichenialesC.Michael Hogan. 2010. [http://www.eoearth.org/article/Fern Fern. Encyclopedia of Earth. National council for Science and the Environment]. Washington, DC appeared as early in the fossil record as the Cretaceous, and achieved an early broad distribution. Gymnosperm taxa like Bennettitales died out before the end of the period. Terrestrial fauna On land, mammals were a small and still relatively minor component of the fauna. Early marsupial mammals evolved in the Early Cretaceous, with true placentals emerging in the Late Cretaceous period. The fauna was dominated by archosaurian reptiles, especially dinosaurs, which were at their most diverse stage. Pterosaurs were common in the early and middle Cretaceous, but as the Cretaceous proceeded they faced growing competition from the adaptive radiation of birds, and by the end of the period only two highly specialized families remained. The Liaoning lagerstätte (Chaomidianzi formation) in China provides a glimpse of life in the Early Cretaceous, where preserved remains of numerous types of small dinosaurs, birds, and mammals have been found. The coelurosaur dinosaurs found there represent types of the group Maniraptora, which is transitional between dinosaurs and birds, and are notable for the presence of hair-like feathers. Insects diversified during the Cretaceous, and the oldest known ants, termites and some lepidopterans, akin to butterflies and moths, appeared. Aphids, grasshoppers, and gall wasps appeared. Image:Tyrannosaurus BW.jpg|''Tyrannosaurus rex, one of the largest land predators of all time, lived during the late Cretaceous. File:Velociraptor dinoguy2.jpg|Up to 2 m-long ''Velociraptor was likely feathered and roamed the late Cretaceous. File:Triceratops BW.jpg|''Triceratops'' is one of the most recognizable genera of the Cretaceous. File:Eomaia NT.jpg|Mammals were a minor part of the Cretacean fauna with Eomaia being the first eutherian. Image:Coloborhynchus piscator jconway.jpg|A pterosaur, Anhanguera piscator Marine fauna In the seas, rays, modern sharks and teleosts became common.http://www.talkorigins.org/origins/geo_timeline.html Marine reptiles included ichthyosaurs in the early and middle of the Cretaceous (becoming extinct during the late Cretaceous Cenomanian-Turonian anoxic event), plesiosaurs throughout the entire period, and mosasaurs appearing in the Late Cretaceous. Baculites, an ammonite genus with a straight shell, flourished in the seas along with reef-building rudist clams. The Hesperornithiformes were flightless, marine diving birds that swam like grebes. Globotruncanid Foraminifera and echinoderms such as sea urchins and starfish (sea stars) thrived. The first radiation of the diatoms (generally siliceous, rather than calcareous) in the oceans occurred during the Cretaceous; freshwater diatoms did not appear until the Miocene.http://www.ucmp.berkeley.edu/mesozoic/cretaceous/cretlife.html The Cretaceous was also an important interval in the evolution of bioerosion, the production of borings and scrapings in rocks, hardgrounds and shells (Taylor and Wilson, 2003). File:Kronosaurus hunt1DB.jpg|A scene from the early Cretaceous: a Woolungasaurus is attacked by a Kronosaurus. File:TylosaurusDB2.jpg|''Tylosaurus'' was the largest known mosasaur, carnivorous marine reptiles that emerged in the late Cretaceous. File:Hesperornis BW.jpg|Strong-swimming and toothed predatory waterbird Hesperornis roamed late Cretacean oceans. Image:DiscoscaphitesirisCretaceous.jpg|The ammonite Discoscaphites iris, Owl Creek Formation (Upper Cretaceous), Ripley, Mississippi. File:The fossils from Cretaceous age found in Lebanon.jpg|A plate with Nematonotus sp., Pseudostacus sp., and a partial Dercetis triqueter, found in Hakel, Lebanon End-Cretaceous extinction event or comet is today widely accepted as the main reason for the Cretaceous–Paleogene extinction event.]] There was a progressive decline in biodiversity during the Maastrichtian stage of the Cretaceous period prior to the suggested ecological crisis induced by events at the K–Pg boundary (K–T boundary). Furthermore, biodiversity required a substantial amount of time to recover from the K–T event, despite the probable existence of an abundance of vacant ecological niches. Despite the severity of this boundary event, there was significant variability in the rate of extinction between and within different clades. Species which depended on photosynthesis declined or became extinct because of the reduction in solar energy reaching the Earth's surface due to atmospheric particles blocking the sunlight. As is the case today, photosynthesizing organisms, such as phytoplankton and land plants, formed the primary part of the food chain in the late Cretaceous. Evidence suggests that herbivorous animals, which depended on plants and plankton as their food, died out as their food sources became scarce; consequently, top predators such as Tyrannosaurus rex also perished. Coccolithophorids and molluscs, including ammonites, rudists, freshwater snails and mussels, as well as organisms whose food chain included these shell builders, became extinct or suffered heavy losses. For example, it is thought that ammonites were the principal food of mosasaurs, a group of giant marine reptiles that became extinct at the boundary. Omnivores, insectivores and carrion-eaters survived the extinction event, perhaps because of the increased availability of their food sources. At the end of the Cretaceous there seem to have been no purely herbivorous or carnivorous mammals. Mammals and birds which survived the extinction fed on insects, larvae, worms, and snails, which in turn fed on dead plant and animal matter. Scientists theorise that these organisms survived the collapse of plant-based food chains because they fed on detritus. In stream communities, few groups of animals became extinct. Stream communities rely less on food from living plants and more on detritus that washes in from land. This particular ecological niche buffered them from extinction. Similar, but more complex patterns have been found in the oceans. Extinction was more severe among animals living in the water column, than among animals living on or in the sea floor. Animals in the water column are almost entirely dependent on primary production from living phytoplankton, while animals living on or in the ocean floor feed on detritus or can switch to detritus feeding. The largest air-breathing survivors of the event, crocodilians and champsosaurs, were semi-aquatic and had access to detritus. Modern crocodilians can live as scavengers and can survive for months without food and go into hibernation when conditions are unfavourable, and their young are small, grow slowly, and feed largely on invertebrates and dead organisms or fragments of organisms for their first few years. These characteristics have been linked to crocodilian survival at the end of the Cretaceous. Image:FaringdonCobble.JPG|Numerous borings in a Cretaceous cobble, Faringdon, England; these are excellent examples of fossil bioerosion. Image:Cretaceous_hardground.jpg|Cretaceous hardground from Texas with encrusting oysters and borings. The scale bar is 10 mm. Image:RudistCretaceousUAE.jpg|Rudist bivalves from the Cretaceous of the Omani Mountains, United Arab Emirates. Scale bar is 10 mm. Image:InoceramusCretaceousSouthDakota.jpg|''Inoceramus'' from the Cretaceous of South Dakota. See also * Chalk Formation * List of fossil sites (with link directory) * South Polar dinosaurs * Western Interior Seaway * Cretaceous Thermal Maximum Notes References * * Neal L Larson, Steven D Jorgensen, Robert A Farrar and Peter L Larson. Ammonites and the other Cephalopods of the Pierre Seaway. Geoscience Press, 1997. * Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006. * Ovechkina, M.N. and Alekseev, A.S. 2005. Quantitative changes of calcareous nannoflora in the Saratov region (Russian Platform) during the late Maastrichtian warming event. Journal of Iberian Geology 31 (1): 149–165. PDF * —detailed coverage of various aspects of the evolutionary history of the insects. * Skinner, Brian J., and Stephen C. Porter. The Dynamic Earth: An Introduction to Physical Geology. 3rd ed. New York: John Wiley & Sons, Inc., 1995. ISBN 0-471-60618-9 * Stanley, Steven M. Earth System History. New York: W.H. Freeman and Company, 1999. ISBN 0-7167-2882-6 * Taylor, P.D. and Wilson, M.A., 2003. Palaeoecology and evolution of marine hard substrate communities. Earth-Science Reviews 62: 1–103.http://www3.wooster.edu/geology/Taylor%26Wilson2003.pdf External links * UCMP Berkeley Cretaceous page * Bioerosion website at The College of Wooster * Cretaceous Microfossils: 180+ images of Foraminifera Category:Cretaceous Category:Mesozoic